1. Field of the Invention
This invention relates to a toner for electrophotography, particularly to a capsule toner suitable for pressure fixing.
2. Description of the Prior Art
Electrophotographic developing methods such as the powder cloud method, the fur brush method, the cascade developing method and the magnetic brush developing method have been known in the art. For the toner to be used in these methods, fine colored powder of a natural or synthetic resin containing a dye or pigment dispersed therein has been used. For example, in the magnetic brush method which is widely practiced in these days, a two-component developer comprising a mixture of iron powder called as the carrier and a toner is employed. In the case of such a two-component developer, for maintaining good development, cumbersome operations are required for maintenance and adjustment to cope with carrier contamination and toner concentration changes. In recent years, the magnetic brush method employing a one-component developer containing fine magnetic particles such as of magnetite incorporated therein has been developed and is now coming to be practically utilized.
As the system for fixing the toner image developed, it is generally practiced to heat and melt the toner by an infrared radiation heater or a heating roller, thereby effecting fusion and solidification onto a supporting medium. However, for the reasons such as prevention of dangers such as fire and saving of power consumed, a pressure fixing system using a rigid body roller is gradually being adopted.
Toners are made of materials which are chosen so as to be adapted for the respective fixing methods, and, generally speaking, a toner applicable for a specific method cannot be used for another mixing method. Particularly, it is impossible to utilize a toner for heat fusion fixing with an infrared radiation heater as a toner for hot roller fixing. Much less, there is no exchangeability at all between the toner for heat fixing and the toner for pressure fixing. Accordingly, toners have been developed which are suitable for individual fixing methods, and the toner for the one-component pressure fixing method is still now investigated for further improvement.
The method to fix toner by pressurization has a number of advantages, such that there is no danger of scorching of the copied sheet, that copying can be done without waiting time after turning on the power source, that high speed fixing is possible and that the fixing device is simple.
For such a pressure-fixable toner, the constituent resin is required to have characteristics suitable for pressure fixing, and the resins suited for this purpose are actively being developed. However, no practical pressure-fixable toner has yet been obtained, which is excellent in pressure fixability, without causing off-set to the pressure rollers, stable in developing and fixing performances during repeated uses, without causing adhesion onto carriers, metal sleeve or the surface of a photosensitive member, and also stable in storage stability without agglomeration or caking during storage. Particularly, with respect to pressure fixability, a problem remains in fixability onto a plain paper.
For example, a pressure-fixable toner comprising a soft material is relatively good in pressure fixability, but it is difficult to make the soft material into toners by micropulverization, and the toner obtained thereby also involves various problems such as tendency to cause off-set to the pressure rollers or other troubles such as adhesion, agglomeration or caking.
On the other hand, a rigid resin is very poor in pressure fixability, although it can be made into toners easily and can readily provide a toner excellent in charging, storage and other characteristics.
For these reasons, various proposals have also been made of toners of the capsule type having a soft material or a liquid as the core material which is enclosed within a shell of a rigid resin.
For example, as disclosed in Japanese Laid-open Patent Application No. 139745/1975, encapsulation is effected by spray-drying a dispersion of the powder of a kneaded product of an aliphatic compound and magnetic powder in a solution of a polymer for forming a coated film dissolved in an organic solvent. However, since an aliphatic compound has a slight solubility in an organic solvent, the resultant capsule involves the drawbacks such that presence of the aliphatic compound of the core material on the surface of the capsule will markedly lower the free flowing property of the toner and also that blocking of the toner occurs during storage.
The microcapsule toner disclosed in U.S. Pat. No. 4,254,201 is prepared by spray-drying with a spray drier a mixture of a pressure-fixable core material coated with a film-forming polymer, additives such as magnetic material, carbon black, etc. and a latex emulsion. However, because the toner thus obtained contains a water-soluble resin or a surfactant contained as a stabilizer of the latex emulsion as such on the toner surface, electric resistance of the toner may be lowered under humid conditions, whereby developing and transfer characteristics may be worsened.